Source Of OH Research Articles

Enhancement of removing OH bonds from low-temperature deposited silicon oxide films by adding water vapor into NH3 gas annealing at 130 oC

Abstract It was found that annealing with water-vapor-added NH3 gas (water-added NH3) is more effective than with dry NH3 at removing residual OH bonds in silicon oxide (SiOx) films deposited by atmospheric chemical vapor deposition with an organic silicon source. Fourier transform infrared spectra showed that reduction amount of OH bonds using the water-added NH3 was ~4 or ~1.3 times larger than using the conventional dry N2 or dry NH3 mixed with N2 gas without water, respectively. This result is something strange because water can be a potential candidate of OH sources. The effect of water vapor on OH-bond removal can be explained by considering following three factors. One is that low-temperature SiOx films are constrained somewhat, the second is that strained Si-O-Si bonds are in higher or more unstable energy state than strain-free ones, and the third is that highly strained bonds are easily hydroxylated to form Si-OH bonds.

Relative humidity driven nocturnal HONO formation mechanism in autumn haze events of Beijing

Nitrous acid (HONO), a key precursor of hydroxyl radicals (OH), is one of the factors affecting atmospheric chemistry and air quality. Currently, the proposed sources of HONO are not able to fully explain observed HONO concentrations. In this study, a comprehensive field observation of HONO was conducted in the autumn of 2021 in urban Beijing. The box model using a default Master Chemical Mechanism (MCM) was unable to reproduce the observed HONO concentrations with a normalized mean bias (NMB) of −92.8%. The NMB improved to −46.1% after the inclusion of seven additional HONO formation pathways. Several factors like vehicle emission factor (1.23%) and nocturnal NO2 heterogeneous uptake coefficient on the ground surface (8.25 × 10−6) were calculated based on observational data. The enhancement factor for nocturnal NO2 heterogeneous conversion was established as a function of relative humidity (RH) and incorporated into the model, which compensated for the missing nocturnal HONO sources and well-reproduced the observed HONO concentrations, with an NMB of −5.1%. The major source of HONO at night was found to be the heterogeneous reaction of NO2 on the ground surface, contributing up to 85.6%. During the daytime, it was the homogeneous reaction of NO with OH, accounting for 41.8%. The daytime primary source of OH was mainly the photolysis of HONO, which constituted 73.6% and therefore promoted the formation of secondary pollutants and exacerbated haze events.

Hydroxide-Source-Dependent Polymorphism and Phase Stability of Cobalt(II) Hydroxides in Diffusion-Driven Systems.

Hydroxides of cobalt(II) exist predominantly in two polymorphic forms, namely, the blue-green α-form [α-Co(OH)2] and reddish β-form [β-Co(OH)2]. These hydroxides have a layered structure with interlayer galleries of around 7 and 4 Å, respectively, for α- and β-Co(OH)2. In most of the previous studies, both the polymorphs were synthesized separately, and a few of them showed that the α-form gets converted to a thermodynamically more stable β-form via physical processes. In the present work, we have optimized the conditions for the simultaneous synthesis of both polymorphs under identical conditions in the same reactor using the 1D reaction-diffusion framework by employing different outer electrolytes. We found that the polymorph chemistry of Co(OH)2 depends on the source and concentration of OH- rather than other reaction conditions or later physical transformation. The products are characterized to confirm their morphology, structure, and chemical environment. We observed that the use of NaOH and NH4OH as the OH- precursor leads to α-Co(OH)2 only; however, with NaOH, a continuous precipitate is formed, and with NH4OH, periodic precipitation is formed. On the other hand, with hydrazine (HYZ) as the OH- source, Liesegang bands of α-Co(OH)2 and β-Co(OH)2 as granules are formed throughout the diffusion reactor. Another intriguing observation on the HYZ system is that at its high concentration, the bands of α-Co(OH)2 get converted to β-Co(OH)2. We articulate the reasons and mechanism of those observations.

A comparative study on the spectral characteristics of nanosecond pulsed discharges in atmospheric He and a He+2.3%H2O mixture

Nanosecond pulsed discharges at atmospheric pressure in a pin-to-pin electrode configuration are well reproducible in time and space, which is beneficial to the fundamentals and applications of low-temperature plasmas. In this experiment, the discharges in helium (He) and He with 2.3% water vapor (H2O) are driven by a series of 10 ns overvoltage pulses (~13 kV). Special attention is paid to the spectral characteristics obtained in the center of discharges by time-resolved optical emission spectroscopy. It is found that in helium, the emission of atomic and molecular helium during the afterglow is more intense than that in the active discharge, while in the He+2.3%H2O mixture, helium emission is only observed during the discharge pulse and the molecular helium emission disappears. In addition, the emissions of OH(A-X) and Hα present similar behavior that increases sharply during the falling edge of the voltage pulse as the electrons cool down rapidly. The gas temperature is set to remain low at 540 K by fitting the OH(A-X) band. A comparative study on the emission of radiative species (He, He2, OH and H) is performed between these two discharge cases to derive their main production mechanisms. In both cases, the dominant primary ion is He+ at the onset of discharges, but their He+ charge transfer processes are quite different. Based on these experimental data and a qualitative discussion on the discharge kinetics, with regard to the present discharge conditions, it is shown that the electron-assisted three-body recombination processes appear to be the significant sources of radiative OH and H species in high-density plasmas.

Unveiling the transformation of 2,4,6-trihalophenols by active species in drinking water pipelines: The role of goethite

The long-term accumulated pipe scale in the water distribution system (WDS) is vital for ensuring the safety of drinking water delivery. This study revealed a novel positive role of pipe scale in triggering interface free radicals for aromatic substances degradation. The goethite (main constituents of pipe scale) converts from the trivalent state to the divalent state by withdraw electrons from aromatic phenolic compounds. Active free radicals such as hydroxyl radical (OH), Cl, and ClO were subsequently formed via a cascade of chain reactions under WDS conditions, among which OH was the dominate active species driving efficient transformation of 2,4,6-trichlorophenol (60.8%) and 2,4,6-tribromophenol (80.5%) and the corresponding toxicity attenuation. The goethite mediated chlorination is a surface reaction following Langmuir–Hinshelwood model. Our works revealed the presence of natural source of OH for disinfection byproducts transformation. Given the ubiquitous existence of iron scales inside ductile iron pipe, the finding renewed the understanding of the impact of WDS on drinking water quality.

A simulation-based approach for understanding CO2 capture and mineralization dynamics in desalination brine

In the pursuit of sustainability, it is crucial to repurpose waste resources into valuable chemicals. One promising avenue is sequential CO2 mineralization using natural seawater or desalination brine solution, which concurrently removes CO2 from the atmosphere while recovering valuable minerals. To realize this approach, it is paramount to grasp the factors influencing the process efficiency and product composition. Here, we present a simulation approach using the visual MINTEQ v3.1 and PHREEQC v3.7.0 models to investigate the impact of CO2 concentration and OH− (as an alkaline source) on the mineral recovery process. The findings reveal that both CO2 concentration and OH− significantly influence the mineral phase composition. Specifically, higher CO2 flow rates or elevated OH− levels promote calcite formation while suppressing the brucite phase, with limited hydromagnesite formation. Moreover, an interactive effect between OH− and CO2 is observed, with OH− essential for carbonate formation. We further validated these insights through preliminary experiments employing electrochemical methods as a source of OH−, wherein aqueous CO2 and applied current density (proportional to OH− concentration) regulated mineral phase composition. Notably, the weight percentage of the brucite phase in the precipitates ranged from 67 % to 73 %, while the calcite phase varied from 20 % to 26 % with alterations to the applied electric currents. The present study not only provides valuable insights into the recovery of potential elements and efficient capture of CO2 in non-synthesized brine solution with a combined theoretical and experimental approach but also demonstrates the viability of employing electrochemical-driven methods as a greener alkaline source for CO2 capture and mineral recovery.

Characterizing Hydroxyl Radical Formation from the Light-Driven Fe(II)-Peracetic Acid Reaction, a Key Process for Aerosol-Cloud Chemistry.

The reaction of peracetic acid (PAA) and Fe(II) has recently gained attention due to its utility in wastewater treatment and its role in cloud chemistry. Aerosol-cloud interactions, partly mediated by aqueous hydroxyl radical (OH) chemistry, represent one of the largest uncertainties in the climate system. Ambiguities remain regarding the sources of OH in the cloud droplets. Our research group recently proposed that the dark and light-driven reaction of Fe(II) with peracids may be a key contributor to OH formation, producing a large burst of OH when aerosol particles take up water as they grow to become cloud droplets, in which reactants are consumed within 2 min. In this work, we quantify the OH production from the reaction of Fe(II) and PAA across a range of physical and chemical conditions. We show a strong dependence of OH formation on ultraviolet (UV) wavelength, with maximum OH formation at λ = 304 ± 5 nm, and demonstrate that the OH burst phenomenon is unique to Fe(II) and peracids. Using kinetics modeling and density functional theory calculations, we suggest the reaction proceeds through the formation of an [Fe(II)-(PAA)2(H2O)2] complex, followed by the formation of a Fe(IV) complex, which can also be photoactivated to produce additional OH. Determining the characteristics of OH production from this reaction advances our knowledge of the sources of OH in cloudwater and provides a framework to optimize this reaction for OH output for wastewater treatment purposes.

A detailed kinetic submechanism for OH[formula omitted] chemiluminescence in hydrogen combustion revisited. Part 1

In this series of two papers, we present a novel physically sound kinetic submechanism aimed at modeling the formation and decay of chemiluminescent (electronically excited) OH∗ molecules in the ignition and combustion of hydrogen and hydrogen-based mixtures over a wide range of thermodynamic parameters and mixture compositions. The present part of this series describes the preparatory stages of the work, such as the aggregation of the data set for the OH∗ chemiluminescent emission (including the OH∗ emission profiles obtained in our shock-tube experiments), the choice of the basic kinetic model of hydrogen oxidation, and, importantly, quantum chemical calculations and theoretical estimates for the rate constant of the nonadiabatic preassociation reaction H + O + M ⇆ OH∗ + M, recognized as a major source of excited OH∗ molecules in hydrogen-air flames. These rate constant estimates are carried out in a wide range of temperatures (200–4000 K) and pressures (10−3−100 bar). It is shown that the rate constant of this key reaction exhibits distinct non-Arrhenius temperature behavior and, moreover, has a complex fall-off pressure dependence, with the transition from a third to a second-order, high-pressure regime occurring at pressures about 10 atm. The obtained dependence on temperature and pressure is in excellent agreement with the known experimental data and can be recommended (as part of the appropriate reaction mechanisms) for further kinetic modeling of OH∗ chemiluminescence accompanying the high-temperature oxidation of hydrogen and other fuels.Novelty and Significance StatementAlthough ultraviolet OH∗ chemiluminescence as an optical signature of the combustion process and the underlying chemistry is known to provide unique capabilities for combustion diagnostics, the development of the detailed reaction mechanisms intended for quantitative interpretation of the OH∗ emission measurements has now almost stagnated. Indeed, existing kinetic models contain a discouragingly small number of elementary processes and, apparently, do not account for the full variety of possible reaction pathways of OH∗ formation and depletion. Therefore, in this serial work, we have revised the current ideas about the kinetics of OH∗ production and consumption during ignition and combustion and focused first on the OH∗ kinetics in a reacting H2/O2/Ar/N2 mixture. The fundamentally significant and novel results of this Part of the present series are (i) the rate constant of the nonadiabatic preassociation reaction H + O + M ⇆ OH∗ + M (the primary source of OH∗ in hydrogen combustion), theoretically obtained for the first time as a function of temperature and pressure, and (ii) the OH∗ emission profiles obtained in our shock-tube experiments. The other aspects described here, such as the aggregation of the literature-based data set for the post-shock OH∗ chemiluminescent emission and the choice of the basic kinetic model of hydrogen oxidation, are also of interest and relevance.

Additional HONO and OH Generation from Photoexcited Phenyl Organic Nitrates in the Photoreaction of Aromatics and NOx.

HONO acts as a major OH source, playing a vital role in secondary pollutant formation to deteriorate regional air quality. Strong unknown sources of daytime HONO have been widely reported, which significantly limit our understanding of radical cycling and atmospheric oxidation capacity. Here, we identify a potential daytime HONO and OH source originating from photoexcited phenyl organic nitrates formed during the photoreaction of aromatics and NOx. Significant HONO (1.56-4.52 ppb) and OH production is observed during the photoreaction of different kinds of aromatics with NOx (18.1-242.3 ppb). We propose an additional mechanism involving photoexcited phenyl organic nitrates (RONO2) reacting with water vapor to account for the higher levels of measured HONO and OH than the model prediction. The proposed HONO formation mechanism was evidenced directly by photolysis experiments using typical RONO2 under UV irradiation conditions, during which HONO formation was enhanced by relative humidity. The 0-D box model incorporated in this mechanism accurately reproduced the evolution of HONO and aromatic. The proposed mechanism contributes 5.9-36.6% of HONO formation as the NOx concentration increased in the photoreaction of aromatics and NOx. Our study implies that photoexcited phenyl organic nitrates are an important source of atmospheric HONO and OH that contributes significantly to atmospheric oxidation capacity.

Reactive aldehyde chemistry explains the missing source of hydroxyl radicals

Hydroxyl radicals (OH) determine the tropospheric self-cleansing capacity, thus regulating air quality and climate. However, the state-of-the-art mechanisms still underestimate OH at low nitrogen oxide and high volatile organic compound regimes even considering the latest isoprene chemistry. Here we propose that the reactive aldehyde chemistry, especially the autoxidation of carbonyl organic peroxy radicals (R(CO)O2) derived from higher aldehydes, is a noteworthy OH regeneration mechanism that overwhelms the contribution of the isoprene autoxidation, the latter has been proved to largely contribute to the missing OH source under high isoprene condition. As diagnosed by the quantum chemical calculations, the R(CO)O2 radicals undergo fast H-migration to produce unsaturated hydroperoxyl-carbonyls that generate OH through rapid photolysis. This chemistry could explain almost all unknown OH sources in areas rich in both natural and anthropogenic emissions in the warm seasons, and may increasingly impact the global self-cleansing capacity in a future low nitrogen oxide society under carbon neutrality scenarios.

Efficient selection of gravitationally lensed OH megamasers with MeerKAT and the Square Kilometre Array

ABSTRACT There has been a recent resurgence in hydroxyl (OH) megamaser research driven by Square Kilometre Array (SKA) precursor/pathfinder telescopes. This will continue in the lead-up to the SKA mid-frequency array, which will greatly expand our view of OH megamasers and their cosmic evolution over ≳80 per cent of the age of the Universe. This is expected to yield large scientific returns as OH megamasers trace galaxy mergers, extreme star formation, high molecular gas densities, and potentially binary/dual supermassive black hole systems. In this paper, we predict the distortion to the OH luminosity function that a magnification bias will inflict, and in turn, predict the distortion on the OH megamaser number counts as a function of redshift. We identify spectral flux density thresholds that will enable efficient lensed OH megamaser selection in large spectral line surveys with MeerKAT and SKA. The surface density of lensed galaxies that could be discovered in this way is a strong function of the redshift evolution of the OH megamaser luminosity function, with predictions as high as ∼1 lensed OH source per square degree at high redshifts (z ≳ 1) for anticipated SKA spectral line survey designs. This could enable efficient selection of some of the most highly obscured galaxies in the Universe. This high-redshift selection efficiency, in combination with the large survey speed of the SKA at ≲1 GHz frequencies and the high magnifications possible with compact OH emission regions (μOH ≫ 10), will enable a transformational view of OH in the Universe.

Field Quantification of Hydroxyl Radicals by Flow-Injection Chemiluminescence Analysis with a Portable Device.

Hydroxyl radical (•OH) is a powerful oxidant abundantly found in nature and plays a central role in numerous environmental processes. On-site detection of •OH is highly desirable for real-time assessments of •OH-centered processes and yet is restrained by a lack of an analysis system suitable for field applications. Here, we report the development of a flow-injection chemiluminescence analysis (FIA-CL) system for the continuous field detection of •OH. The system is based on the reaction of •OH with phthalhydrazide to generate 5-hydroxy-2,3-dihydro-1,4-phthalazinedione, which emits chemiluminescence (CL) when oxidatively activated by H2O2 and Cu3+. The FIA-CL system was successfully validated using the Fenton reaction as a standard •OH source. Unlike traditional absorbance- or fluorescence-based methods, CL detection could minimize interference from an environmental medium (e.g., organic matter), therefore attaining highly sensitive •OH detection (limits of detection and quantification = 0.035 and 0.12 nM, respectively). The broad applications of FIA-CL were illustrated for on-site 24 h detection of •OH produced from photochemical processes in lake water and air, where the temporal variations on •OH productions (1.0-12.2 nM in water and 1.5-37.1 × 107 cm-3 in air) agreed well with sunlight photon flux. Further, the FIA-CL system enabled field 24 h field analysis of •OH productions from the oxidation of reduced substances triggered by tidal fluctuations in coastal soils. The superior analytical capability of the FIA-CL system opens new opportunities for monitoring •OH dynamics under field conditions.

Observational Evidence of Unknown NOx Source and Its Perturbation of Oxidative Capacity in Bermuda's Marine Boundary Layer

AbstractNitrogen oxides (NOx) are key intermediates in the atmospheric cycling of reactive nitrogen, the spatiotemporal distribution of which modulates ozone (O3) production. Field campaigns were conducted at the Tudor Hill Marine Atmospheric Observatory, Bermuda, in the spring and summer of 2019 to explore atmospheric cycling of NOx and its modulation of photochemical O3 production in the marine boundary layer. In aged, clean marine air, an atypical NO2 diel profile with a solar noon peak of 69 ± 5 pptv was recorded, challenging the classic U‐shaped diel profile with a solar noon valley characterized by fast photolysis and oxidation consumption in the daytime. This result indicated an unknown daytime NOx source excluded from the current near‐explicit chemical model, which underestimated the solar noon NOx level by 20–56 pptv and source rate by 9.7–33.5 pptv hr−1, considering the upper and lower limits of total OH reactivity and halogen photochemistry in the marine boundary layer. The observed HONO level accounted for ∼56% of the unknown NOx source, implying an unknown NOx regeneration pathway with HONO as an intermediate. The photochemical nature of the unknown NOx source maximized perturbation of photochemical OH and O3 production. The O3 abundance and production rate were underestimated by 2–4 ppbv and 28%–80%, respectively, and the OH abundance and source rate were 7%–55% and 21%–57% lower than the estimated levels with the constraint of NOx, respectively. The unknown NOx source requires urgent revision of the current understanding of reactive nitrogen cycling and the oxidative capacity of the clean marine atmosphere.

Atmospheric photooxidation and ozonolysis of sabinene: reaction rate coefficients, product yields, and chemical budget of radicals

Abstract. The oxidation of sabinene by the hydroxyl radical (OH) and ozone (O3) was investigated under atmospherically relevant conditions in the atmospheric simulation chamber SAPHIR (Simulation of Atmospheric Photochemistry In a Large Reaction Chamber) at Forschungszentrum Jülich, Germany. The rate coefficients of the reactions of sabinene with OH and with O3 were determined. The temperature dependence between 284 to 340 K of the rate coefficient of the reaction of sabinene with OH, kSAB+OH, was measured for the first time using an OH reactivity instrument, resulting in an Arrhenius expression of (1.67 ± 0.16) × 10−11 × exp((575 ± 30)/T) cm3 s−1. The values agree with those determined in chamber experiments in this work and reported in the literature for ∼ 298 K within the uncertainties of measurements. The ozonolysis reaction rate coefficient of sabinene (kSAB+O3) determined in chamber experiments at a temperature of (278 ± 2) K is (3.4 ± 0.8) × 10−17 cm3 s−1, which is 58 % lower than the value reported in the literature for room temperature. The measurement of products from the oxidation of sabinene by OH resulted in an acetone yield of (21 ± 15) %, a formaldehyde yield of (46 ± 25) %, and a sabinaketone yield of (18 ± 16) %. All yields determined in the chamber experiments agree well with values from previous laboratory studies within their uncertainties. In addition, the formaldehyde yield determined in this study is consistent with that predicted by the sabinene OH-oxidation mechanism which was devised from quantum chemical calculations by Wang and Wang (2018), whereas the acetone yield is about 15 % higher than that predicted by the mechanism. In the ozonolysis experiments, the analysis of product measurements results in an acetone yield of (5 ± 2) %, a formaldehyde yield of (48 ± 15) %, a sabinaketone yield of (31 ± 15) %, and an OH radical yield of (26 ± 29) %. The OH radical yield is lower than expected from the theoretical mechanism in Wang and Wang (2017), but the value still agrees within the uncertainty. An analysis of the chemical budget of OH radicals was performed for the chamber experiments. The analysis reveals that the destruction rate of the OH radical matches the production rate of OH, suggesting that there is no significant missing OH source for example from isomerization reactions of peroxy radicals for the experimental conditions in this work.

Experimental and kinetic modeling studies on oxidation of methanol and di-tert-butyl peroxide in a jet-stirred reactor

Methanol is a potential carbon neutral fuel, but it faces the challenge of ignition difficulties in engines. Di-tert-butyl peroxide (DTBP), a cetane number improver, is considered to be added into methanol to improve ignition. However, there is currently no research of experiments and simulations on the kinetic model of methanol and DTBP. Therefore, the oxidation characteristics of methanol and DTBP were experimentally studied in a jet-stirred reactor under temperatures of 400–1000 K and pressures of 1.03 atm, with equivalence ratios of 0.5, 1 and 2 for pure methanol, pure DTBP, blended fuel (97 % CH3OH + 3 % DTBP; 94 % CH3OH + 6 % DTBP). Then, a new reduced CH3OH-DTBP mechanism containing 75 species and 443 reactions was proposed, which was widely validated by experimental data of ignition delay times, laminar flame speeds and species concentration profiles. Next, the simulations were conducted with CHEMKIN PRO to analyze the effect of DTBP on methanol. The major findings are as follows. After adding DTBP, methanol can react at a lower temperature (550 K), which is due to CH3 generated by decomposition of DTBP promotes the enrichment of OH, thereby promoting the oxidation of methanol. The source of OH at 650 K is CH3 → CH3O2 → CH3O2H → CH3O. At 850 K, the early OH comes from R27 (CH3 + HO2=CH3O + OH), while the later OH comes from the decomposition of H2O2. When the temperature is above 850 K, the addition of DTBP has only a slight effect on the oxidation of methanol. As the temperature increases, the effect of DTBP on shortening the ignition delay time weakens. As DTBP concentration increases, the effect also shows a weakening trend. At low temperatures, the contribution of thermal effects caused by DTBP is greater. As DTBP concentration increases, the proportion of thermal effect contribution increases.

Assessment of Fenton systems based on metabisulphite as a low-cost alternative to hydrogen peroxide

Here we show that a Fenton-like system based on zero-valent iron and metabisulphite (ZVI-MBS) is able to effectively degrade both ibuprofen (a non-steroidal anti-inflammatory drug) and its toxic product 4-isobutylacetophenone in 30 min at pH 3. Based on scavenging experiments, the main reactive species involved in the degradation process would be •OH and SO4•−, with SO4•− likely acting as •OH source. Compared to well-known ZVI-H2O2, ZVI-MBS loses performance considerably at pH > 3. Furthermore, differently from ZVI-H2O2 where the ZVI surface plays major role, dissolved Fe species are important in ZVI-MBS. In presence of MBS, it would thus be more convenient to add Fe2+ directly in the form of ferrous salts than to use ZVI as source of dissolved iron. Fe2+-MBS, similarly to ZVI-MBS, experiences considerable drop in performance above pH 3. MBS is much cheaper, more stable, and safer to operate than H2O2. However, the need to treat water at pH 3 has the important drawback that conductivity/salinity is increased by water acidification for treatment and neutralisation after treatment. That would be appropriate in most cases for water discharge, but could limit reuse of treated water in some fields such as irrigation.

Snowpack nitrate photolysis drives the summertime atmospheric nitrous acid (HONO) budget in coastal Antarctica

Abstract. Measurements of atmospheric nitrous acid (HONO) amount fraction and flux density above snow were carried out using a long-path absorption photometer at Halley station in coastal Antarctica between 22 January and 3 February 2022. The mean ±1σ HONO amount fraction was (2.1 ± 1.5) pmol mol−1 and showed a diurnal cycle (range of 1.0–3.2 pmol mol−1) with a maximum at solar noon. These HONO amount fractions are generally lower than have been observed at other Antarctic locations. The flux density of HONO from the snow, measured between 31 January and 1 February 2022, was between 0.5 and 3.4×1012 m-2s-1 and showed a decrease during the night. The measured flux density is close to the calculated HONO production rate from photolysis of nitrate present in the snow. A simple box model of HONO sources and sinks showed that the flux of HONO from the snow makes a >10 times larger contribution to the HONO budget than its formation through the reaction of OH and NO. Ratios of these HONO amount fractions to NOx measurements made in summer 2005 are low (0.15–0.35), which we take as an indication of our measurements being comparatively free from interferences. Further calculations suggest that HONO photolysis could produce up to 12 pmolmol-1h-1 of OH, approximately half that produced by ozone photolysis, which highlights the importance of HONO snow emissions as an OH source in the atmospheric boundary layer above Antarctic snowpacks.

Photochemical Implications of Changes in the Spectral Properties of Chromophoric Dissolved Organic Matter: A Model Assessment for Surface Waters.

Chromophoric dissolved organic matter (CDOM) is the main sunlight absorber in surface waters and a very important photosensitiser towards the generation of photochemically produced reactive intermediates (PPRIs), which take part in pollutant degradation. The absorption spectrum of CDOM (ACDOM(λ), unitless) can be described by an exponential function that decays with increasing wavelength: ACDOM(λ) = 100 d DOC Ao e-Sλ, where d [m] is water depth, DOC [mgC L-1] is dissolved organic carbon, Ao [L mgC-1 cm-1] is a pre-exponential factor, and S [nm-1] is the spectral slope. Sunlight absorption by CDOM is higher when Ao and DOC are higher and S is lower, and vice versa. By the use of models, here we investigate the impact of changes in CDOM spectral parameters (Ao and S) on the steady-state concentrations of three PPRIs: the hydroxyl radical (•OH), the carbonate radical (CO3•-), and CDOM excited triplet states (3CDOM*). A first finding is that variations in both Ao and S have impacts comparable to DOC variations on the photochemistry of CDOM, when reasonable parameter values are considered. Therefore, natural variability of the spectral parameters or their modifications cannot be neglected. In the natural environment, spectral parameters could, for instance, change because of photobleaching (prolonged exposure of CDOM to sunlight, which decreases Ao and increases S) or of the complex and still poorly predictable effects of climate change. A second finding is that, while the steady-state [3CDOM*] would increase with increasing ACDOM (increasing Ao, decreasing S), the effect of spectral parameters on [•OH] and [CO3•-] depends on the relative roles of CDOM vs. NO3- and NO2- as photochemical •OH sources.

A Profile-based Approach to Finding New Water Fountain Candidates using Databases of Circumstellar Maser Sources

AbstractWater fountains (WFs) are thought to represent an early stage in the morphological evolution of circumstellar envelopes surrounding low- and intermediate-mass evolved stars. These objects are considered to transition from spherical to asymmetric shapes. Despite their potential importance in this transformation process of evolved stars, there are only a few known examples. To identify new WF candidates, we used databases of circumstellar OH (1612 MHz) and H2O (22.235 GHz) maser sources, and compared the velocity ranges of the two maser lines. Finally, 41 sources were found to have a velocity range for the H2O maser line that exceeded that of the OH maser line. Excluding known planetary nebulae and after reviewing the maser spectra in the original literature, we found for 11 sources the exceedance as significant, qualifying them as new WF candidates.

Measurements and Modelling of OH and Peroxy Radicals in an Indoor Environment Under Different Light Conditions and VOC Levels

Indoor measurements of OH and sum of peroxy radicals XO2=HO2+RO2 were conducted in a room using window glasses of different transparencies and applying different coatings to the walls. Average OH and XO2 concentrations were found to vary in the range (0.6-4)×105moleculecm−3 and (1-7)×107moleculecm−3 respectively with anti-UV windows, and (6-10)×105moleculecm−3 and (4-16)×107moleculecm−3 respectively with borosilicate glasses. The OH and XO2 concentrations were compared with simulation results obtained using the H2I model, which accounts for the mixing between the sunlit and shaded volumes of the room. Taking into account the measurement uncertainty, the simulated OH concentrations agree with the observations on average while the simulated XO2 concentrations tend to be overestimated. Based on the model results, ozonolysis of unsaturated VOCs and photolysis of HONO and of organic compounds are found to represent the main primary sources of OH and XO2 radicals, the latter being more important in the sunlit volume. Despite the lower rate of the radical initiation in the shaded volume, the difference of radical concentrations in the shaded and sunlit volumes is mitigated by an interplay between the mixing time and the lifetime of XO2 radicals, allowing efficient transport of XO2 radicals into the shaded volume and acting there as a source of OH via radical propagation processes.